Thermal flow measurement rests essentially on two measuring principles, the thermal dispersion measuring principle and the thermal profile, or temperature rise, measuring principle. In the case of thermal dispersion, a heated measuring element is exposed to the flow of the medium being measured. The cooling rate caused thereby is a measure for the flow velocity. In the case of measuring by means of thermal profile, or temperature rise, in a limited region of the flow, heat is introduced, whereby the temperature increases locally, from which, in turn, taking into consideration the supplied energy, the mass flow can be calculated. In such case, two temperature sensors measure the temperatures of the medium at two different points, most often, before and after the supplied heat. Also, a number of heating elements and temperature sensors can be applied, in order to obtain a better picture of the thermal profile.
In the case of both measuring methods, the sensors can be placed in the main line or in a bypass. The two principles of functioning overlap significantly in practice.
Conventional thermal, flow measuring devices for industrial processes use usually two, as much as possible equally embodied, temperature sensors, which are arranged in, most often, pin-shaped, metal sleeves, so-called stingers, and which are in thermal contact with the medium flowing through a measuring tube or through the pipeline. In such case, the two temperature sensors are usually installed in a measuring tube; the temperature sensors can, however, also be mounted directly in the pipeline. One of the two temperature sensors is a so-called active temperature sensor, which is heated by means of a heating unit. Provided as heating unit is either an additional resistance heating element, or the temperature sensor itself is a resistance element, e.g. an RTD (Resistance Temperature Device) sensor, which is heated by conversion of electrical power, e.g. by a corresponding variation of the measuring electrical current. Recently, also thin film, resistance elements, so-called Thin Film Resistance Temperature Devices (TFRTD) are used. The second temperature sensor is a passive temperature sensor and measures the temperature of the medium.
Usually, in a thermal, flow measuring device, the heatable temperature sensor is so heated, that a fixed temperature difference is established between the two temperature sensors. Alternatively, it is also known to supply a constant heating power via a control unit (open, or closed, loop control).
If there is no flow in the measuring tube, then an amount of heat constant with respect to time is required to maintain the predetermined temperature difference. If, in contrast, the medium being measured is moving, then the cooling of the heated temperature sensor is essentially dependent on the mass flow of the medium flowing past. Since the medium is colder than the heated temperature sensor, heat is transported away from the heated temperature sensor by the flowing medium. In order, thus, in the case of a flowing medium, to maintain the fixed temperature difference between the two temperature sensors, an increased heating power is required for the heated temperature sensor. The increased heating power is a measure for the mass flow, i.e. for the mass flow of the medium, through the pipeline.
If, in contrast, a constant heating power is fed in, then the temperature difference between the two temperature sensors lessens as a result of the flow of the medium. The particular temperature difference is then a measure for the mass flow of the medium through the pipeline, or through the measuring tube.
There is, thus, a functional relationship between the heating energy needed for heating the temperature sensor and the mass flow through a pipeline, or through a measuring tube. The dependence of the heat transfer coefficient on the mass flow of the medium through the measuring tube, or through the pipeline, is utilized in thermal, flow measuring devices for determining the mass flow. Devices, which operate on this principle are available from the assignee under the marks, ‘tswitch’, ‘ttrend’ or ‘tmass’.
Thermal flow measuring devices are suited especially for flow measurement of gases, or gas mixtures. If, now, a gas, or a part of a gas, condenses or desublimes on the heated temperature sensor of the thermal, flow measuring device, the measurement signal of the thermal, flow measuring device changes. A measurement peak occurs, and, in the case of an uncorrected measurement signal, an error results in the measured flow. The heat transfer of liquid or solid to temperature sensor is different from the heat transfer of gas to temperature sensor. Through the condensate, or the desublimate, additional heat is withdrawn from the thermal, flow measuring device. The thermal, flow measuring device readjusts, i.e. the heating power, and, therewith, the heat energy, supplied to the medium rises. On the basis of these reactions, the thermal, flow measuring device gives a flow, which does not correspond to the flow of the gas flowing through the measurement line. There is an error in the flow signal. To be viewed as another error source is the occurring heat of condensation, or desublimation, as the case may be. Thus, an aggregate state change of the medium being measured, or of a part of the medium being measured, with the thermal, flow measuring device can be classified as a disturbance. Therefore, it is, to this point in time, only possible, to determine and/or to monitor the flow of dry gas highly accurately with a thermal, flow measuring device.